1. Field of the Invention
The present invention relates to a photovoltaic apparatus and to a mass-producing apparatus for mass-producing spherical semiconductor particles, suitable for manufacture of photovoltaic apparatus and the like.
In the disclosure herein described, the term xe2x80x9cpin junctionxe2x80x9d is to be construed as including a structure that n-, I- and p-type semiconductor layers are formed on an approximately spherical photoelectric conversion element so as to be arranged in this order outward from the inside of the approximately spherical photoelectric conversion element or inward from the outside.
2. Description of the Related Art
A typical related art technique provides a photovoltaic apparatus comprising a photoelectric conversion element composed of a crystal silicon semiconductor wafer. The related art photovoltaic apparatus is of cost expenditure because the step for producing a crystal is complex. Furthermore, the step for manufacturing a semiconductor wafer is not only complex because it includes cutting of a bulk single crystal, slicing, and polishing, but also the step is wasteful because crystal waste produced by the cutting, slicing, polishing etc. amounts to about 50% by volume or more of the original bulk single crystal.
Another related art technique provides a photovoltaic apparatus comprising a photoelectric conversion element composed of an amorphous silicon (abbreviated as xe2x80x9ca-Sixe2x80x9d) thin film, which solves the above-mentioned problems. Since a thin-film photoelectric conversion layer is formed by the plasma CVD (chemical vapor deposition) method, this related art photovoltaic apparatus has advantages that the steps which are conventionally required, such as cutting of a bulk single crystal, slicing, and polishing, are not necessary and that a deposited film can be used in its entirety as device active layers. The amorphous silicon photovoltaic apparatus, however, has a drawback that the semiconductor has a number of crystal defects (i.e., gap states) inside the semiconductor due to the amorphous structure, the amorphous silicon solar battery has a problem that the photoelectric conversion efficiency decreases due to a photo-induced deterioration phenomenon. To solve this problem, conventionally, a technique of inactivating crystal defects by applying hydrogenation treatment has been developed, whereby the manufacture of such electronic devices as an amorphous silicon solar battery has been realized.
Even such a treatment, however, cannot entirely eliminate the adverse effects of crystal defects, and for example, the amorphous silicon solar battery still has a weak point that the photoelectric conversion efficiency decreases by 15% to 25%.
A recently developed technique for suppressing the photo-induced deterioration has realized a stack-type solar battery in which a photoelectrically active i-type layer is made extremely thin and 2-junction or 3-junction solar cells are used, and has succeeded in suppressing the photo-induced deterioration to about 10%. It has become apparent that the degree of photo-induced deterioration decreases when the operation temperature of solar cells is high. Although a module technique in which solar cells are caused to operate in such a condition is now being developed, it does not satisfy all the requirements and further improvements are required.
Still another related art technique in which the above problem is eliminated is disclosed in Japanese Examined Patent Publication JP-B27-54855 (1995). According to this related art technique, a solar array is formed in the following manner. Spherical particles each having a p-type silicon sphere and an n-type silicon skin are buried in a flat sheet of aluminum foil having holes. The internal p-type silicon spheres are exposed by etching away the n-type silicon skins from the back side of the aluminum foil. The exposed-type silicon spheres are connected to another sheet of aluminum foil.
In this related art technique, to reduce the costs by decreasing the used amount of high-purity silicon, it is necessary to decrease the average thickness of the entire device by decreasing the outer diameter of the particles. To increase the conversion efficiency, it is necessary to enlarge the light-receiving surface, and to this end, it is necessary to arrange the particles closer to each other. In summary, a number of particles having a small outer diameter need to be arranged densely and connected to the sheets of aluminum foil. This makes complex the step of connecting the particles to the sheets of aluminum foil, with the result that a sufficient cost reduction is not achieved.
Such spherical semiconductor particles are required in order to manufacture a solar array such as one disclosed in JP-B2 7-54855. In such a solar array, photoelectromotive force generated by applying light to silicon spherical semiconductor particles can be obtained by electrically connecting the silicon spherical semiconductor particles to the metal foil matrix.
As disclosed in U.S. Pat. No. 5,012,619, for example, such spherical particles are manufactured in such a manner that a solid-state material is crushed into particles having irregular outer shapes, resulting particles are put in a cylinder that is provided with a lining for grinding, and a gas eddy flow is formed in the cylinder to thereby collide the particles with the lining or with each other.
This related art technique requires much time and labor to manufacture spherical semiconductor particles and hence is inferior in cost reduction aspect.
Yet another related art technique is disclosed in Japanese Unexamined Patent Publication JP-A 8-239298 (1996). In this related art technique, a thin silicon rod is manufactured in the following manner. A tip portion of a silicon rod that is vertically held is melted by high-frequency heating. After a seed silicon crystal is fusion-bonded to the molten silicon rod, the seed silicon crystal and the silicon rod are moved away from each other in the vertical direction, whereby a thin silicon rod that is less than 1 mm in thickness is obtained. According to this prior art technique, a thin silicon rod is manufactured at a rate of 5 mm/min to 10 mm/min, for example. It is desired that a large number of spherical semiconductor particles be manufactured at much higher rate.
An object of the present invention is to provide a highly reliable, highly efficient photovoltaic apparatus that can be mass-produced easily while the used amount of semiconductor material such as high-purity silicon is decreased, that is, a highly reliable, high-efficiency photovoltaic apparatus that can be manufactured at low costs with smaller amounts of consumption of resources and energy.
Another object of the invention is to provide an apparatus capable of mass-producing spherical semiconductor particles easily by simple manipulations.
A first aspect of the invention provides a photovoltaic apparatus comprising:
(a) a plurality of photoelectric conversion elements, each being of an approximately spherical shape and including a first semiconductor layer and a second semiconductor layer which is located outside the first semiconductor layer, for generating photoelectromotive force between the first and second semiconductor layers, the second semiconductor layer having an opening through which a portion of the first semiconductor layer is exposed; and
(b) a support including a first conductor, a second conductor, and an insulator disposed between the first and second conductors for electrically insulating the first and second conductors from each other, the support having a plurality of recesses which are arranged adjacent to each other and of which inside surfaces are constituted by the first conductor or a coating formed thereon, the photoelectric conversion elements being disposed in the respective recesses so that the photoelectric conversion elements are illuminated with light reflected by part of the first conductor or coating formed thereon which constitutes the recess, the first conductor being electrically connected to the second semiconductor layers of the photoelectric conversion elements, and the second conductor being electrically connected to the exposed portions of the first semiconductor layers.
According to the invention, the approximately spherical photoelectric conversion elements are disposed in the respective recesses of the support and the inside surfaces of the respective recesses are constituted by the first conductor or the coating formed on the first conductor. Therefore, external light such as sunlight is directly applied to each of the photoelectric conversion elements as well as after being reflected by the part of the first conductor or coating formed on the first conductor which is the inside surface of the recess.
Since the photoelectric conversion elements are disposed in the respective recesses, intervals are formed in between, that is, their arrangement is not dense. However, the number of photoelectric conversion elements used is decreased, with the result that the used amount of high-purity material (e.g., silicon) of the photoelectric conversion elements can be reduced and the step of connecting the photoelectric conversion elements to the conductors of the support can be made easier.
Further, the recesses are arranged adjacent to each other, whereby external light is reflected by the inside surfaces of the recesses and then applied to the photoelectric conversion elements. Therefore, external light can be efficiently used for generation of photoelectromotive force by the photoelectric conversion elements. Accordingly it is achieved to maximize the electric power generation amount per unit area opposed to a light source of the photoelectric conversion elements.
The photoelectric conversion elements may be made of a single-crystal, polycrystalline, or amorphous material and may be made of a silicon material, a compound semiconductor material, or the like. The photoelectric conversion elements may have a pn structure, a pin structure, a Schottky barrier structure, a MIS (metal-insulator-semiconductor) structure, a homojunction structure, a heterojunction structure, or the like.
The inside first semiconductor layer is partially exposed through the opening of the outside second semiconductor layer, which makes it possible to take out photoelectromotive force that is generated between the first and second semiconductor layers during application of light. The second semiconductor layers of the respective photoelectric conversion elements disposed in the respective recesses of the support are electrically connected to the first conductor of the support. The exposed portions of the inside first semiconductor layers of the respective photoelectric conversion elements are electrically connected to the second conductor which is formed on the first conductor with the insulator interposed in between. In a structure in which the first conductor and the second conductor extend to form a plane, the photoelectric conversion elements are connected to each other in parallel with the first and second conductors and can produce a large current.
The photoelectric conversion element may either be a complete sphere or have an outer surface that is approximately a complete spherical surface. The first semiconductor layer may be solid and have an approximately spherical shape. Alternatively, the first semiconductor layer may be formed on the outer surface of a core that is prepared in advance. As a further alternative, the approximately spherical first semiconductor layer may have a hollow central portion.
In the invention it is preferable that the photoelectric conversion elements have an outer diameter of 0.5 mm to 2.0 mm.
According to the invention, the photoelectric conversion elements may have an outer diameter of 0.5 mm to 2.0 mm, preferably of 0.8 to 1.2 mm, more preferably of about 1.0 mm. This makes it possible to sufficiently reduce the used amount of material such as high-purity silicon and to maximize the electric power generation amount. Further, the spherical photoelectric conversion elements can be handled easily during manufacture and the productivity can be increased.
In the invention it is preferable that the opening of the second semiconductor layer has a central angle 01 of 45xc2x0 to 90xc2x0.
By setting the central angle xcex81 at 45xc2x0 to 90xc2x0, preferably at 60xc2x0 to 90xc2x0, the amounts of those parts of the first and second semiconductor layers which are discarded to form the opening can be reduced, that is, the loss of material can be reduced. Further, setting the central angle xcex81 in such a range allows the opening to have a sufficient area for the electrical connection between the first semiconductor layer and the second conductor of the support.
In the invention it is preferable that the recesses of the support have respective openings of a polygon (e.g., honeycomb polygon) of which ones adjacent to each other are continuous, that each of the recesses narrows toward a bottom thereof, and the first semiconductor layer and second semiconductor layer of each of the photoelectric conversion elements are electrically connected to the second conductor and the first conductor, respectively, at the bottom or in a vicinity thereof of the recess.
In the invention it is preferable that the first conductor is provided with a circular first connection hole formed at the bottom or in a vicinity thereof of the recess and the insulator is provided with a circular second connection hole having a common axial line with the first connection hole, that a portion of the photoelectric conversion element in a vicinity of the opening of the second semiconductor layer fits in the first connection hole and an outer surface portion above the opening of the second semiconductor layer is electrically connected to an end face of the first connection hole of the first conductor or to a portion thereof in the vicinity of the end face, and that the exposed portion of the first semiconductor layer of the photoelectric conversion element is electrically connected to the second conductor through the second connection hole.
In the invention it is preferable that an outer diameter D1 of the photoelectric conversion elements, an inner diameter D2 of the openings of the second semiconductor layers, and an inner diameter D3 of the first connection holes, and an inner diameter D4 of the second connection holes satisfy a relationship D1 greater than D3 greater than D2 greater than D4.
According to the invention, a portion of the photoelectric conversion element in the vicinity of the opening fits in the first connection hole of the first conductor and the exposed portion of the first semiconductor layer of the photoelectric conversion element is electrically connected to the second conductor through the second connection hole of the insulator of the support, the first conductor and the second conductor of the support can easily be connected electrically to the second semiconductor layer and the first semiconductor layer, respectively, of the photoelectric conversion element.
As for the electrical connection between the second semiconductor layer and the first conductor, a portion, above the opening, of the outer surface of the second semiconductor layer is electrically connected to the end face of the first connection hole and/or a portion of the first conductor in the vicinity of the end face, that is, the inner circumferential face of the first connection hole and/or a portion of the first conductor in the vicinity of and surrounding the first connection hole (see FIG. 1).
For example, each projection of the second conductor formed by elastic deformation may be inserted through the second connection hole and electrically connected to the portion of the first semiconductor layer that is exposed through the opening. Alternatively, the portion of the first semiconductor layer that is exposed through the opening may be connected to the second conductor with conductive paste provided in the second connection hole or via a conductive bump such as a metal bump.
Setting the outer diameter D1 and the inner diameters D2-D4 so as to satisfy the above inequality enables reliable electrical connection without causing undesired short-circuiting.
In the invention it is preferable that a light-gathering ratio x which equals to S1/S2 is selected to be in a range of 2 to 8, wherein S1 is an opening area of each of the recesses of the support and S2 is an area of a cross-section of the photoelectric conversion elements including a center thereof.
A second aspect of the invention provides a photovoltaic apparatus comprising:
(a) a plurality of photoelectric conversion elements, each being of an approximately spherical shape and including a first semiconductor layer and a second semiconductor layer which is located outside the first semiconductor layer, for generating photoelectromotive force between the first and second semiconductor layers, the second semiconductor layer having an opening through which a portion of the first semiconductor layer is exposed; and
(b) a support including a first conductor, a second conductor, and an insulator disposed between the first and second conductors for electrically insulating the first and second conductors from each other, the support having a plurality of recesses which are arranged adjacent to each other and of which inside surfaces are constituted by the first conductor or a coating formed thereon, the photoelectric conversion elements being disposed in the respective recesses so that the photoelectric conversion elements are illuminated with light reflected by part of the first conductor or coating formed thereon which constitutes the recess, the first conductor being electrically connected to the second semiconductor layers of the photoelectric conversion elements, and the second conductor being electrically connected to the exposed portions of the first semiconductor layers,
wherein each of the photoelectric conversion elements has an outer diameter of 0.5 mm to 2 mm, and a light-gathering ratio x which equals to S1/S2 is selected to be in a range of 2 to 8, wherein S1 is an opening area of each of the recesses of the support and S2 is an area of a cross-section of the photoelectric conversion elements including a center thereof.
A third aspect of the invention provides a photovoltaic apparatus comprising:
(a) a plurality of photoelectric conversion elements, each being of an approximately spherical shape and including a first semiconductor layer and a second semiconductor layer which is located outside the first semiconductor layer, for generating photoelectromotive force between the first and second semiconductor layers, the second semiconductor layer having an opening through which a portion of the first semiconductor layer is exposed; and
(b) a support including a first conductor, a second conductor, and an insulator disposed between the first and second conductors for electrically insulating the first and second conductors from each other, the support having a plurality of recesses which are arranged adjacent to each other and of which inside surfaces are constituted by the first conductor or a coating formed thereon, the photoelectric conversion elements being disposed in the respective recesses so that the photoelectric conversion elements are illuminated with light reflected by part of the first conductor or coating formed thereon which constitutes the recess, the first conductor being electrically connected to the second semiconductor layers of the photoelectric conversion elements, and the second conductor being electrically connected to the exposed portions of the first semiconductor layers, wherein each of the photoelectric conversion elements has an outer diameter of 0.8 mm to 1.2 mm, and a light-gathering ratio x which equals to S1/S2 is selected to be in a range of 4 to 6, wherein S1 is an opening area of each of the recesses of the support and S2 is an area of a cross-section of the photoelectric conversion elements including a center thereof.
For example, the openings of the respective recesses of the support assume honeycomb polygons such as hexagon. Each recess narrows toward its bottom, and a photoelectric conversion element is disposed at the bottom of each recess. Each photoelectric conversion element is electrically connected to the conductors of the support at the bottom or its neighborhood of the recess. Since the openings of the respective recesses assume polygons and are continuous with each other, all the light received by all the surface of the support opposed to a light source (e.g., sunlight) excluding the areas of the photoelectric conversion elements can be applied to the photoelectric conversion elements. Therefore, what is called a light-gathering-type photoelectric conversion element can be realized in which the light-gathering ratio x=S1/S2 is set at 2 to 8, for example (preferably 4 to 6). This makes it possible to increase the intervals between the photoelectric conversion elements, decrease the number of photoelectric conversion elements, and simplify the step of electrically connecting the photoelectric conversion elements to the support. Therefore, the used amount of high-purity semiconductor as a material of the photoelectric conversion elements can be reduced and the invention can be practiced inexpensively. Having a relatively simple structure, the support is superior in productivity and can be manufactured easily.
An experiment conducted by the present inventor shows that when a photovoltaic apparatus according to the invention in which the approximately spherical silicon photoelectric conversion elements have an outer diameter of 800 xcexcm to 1,000 xcexcm and the light-gathering ratio x is set at 4-6 is converted into an imaginary flat plate that uses the same weight of silicon as the silicon that constitutes all the photoelectric conversion elements of the photovoltaic apparatus and that has the same area as the area of a projection of the photovoltaic apparatus onto a plane perpendicular to light coming from a light source, the imaginary flat plate has a thickness of about 90 xcexcm to 120 xcexcm. This means that the amount of silicon that is used for generating an electric power of 1 W is as small as 2 g, which is an epoch-making conclusion. In the above-described first prior art technique using photoelectric conversion elements formed on a single-crystal silicon semiconductor wafer, the silicon single crystal is as thick as 350 xcexcm to 500 xcexcm and the thickness amounts to about 1 mm when the slice loss is included. Therefore, in the first prior art technique, the amount of silicon that is used for generating an electric power of 1 W is about 15 to 20 g. It is understood that the invention can make the used amount of silicon much smaller than in the first prior art technique.
When the light-gathering ratio x is set larger than 8, the number of necessary photoelectric conversion elements can be decreased and the amount of silicon used for generating an electric power of 1 W could further be reduced. However, in practice, as the light-gathering ratio x increases, the light-gathering efficiency which is the ratio of optical energy absorbed by the photoelectric conversion elements to that incident on the recesses becomes smaller and the performance of the photovoltaic apparatus lowers accordingly.
By setting, as described above, the outer diameter of each photoelectric conversion element at 0.5 mm to 2.0 mm (preferably 0.8 mm to 1.2 mm) and setting the light-gathering ratio x at 2 to 8 (preferably 4 to 6), the number of photoelectric conversion elements can be decreased, the amount of silicon used for generating an electric power of 1 W can be reduced, and the step of electrically connecting the photoelectric conversion elements to the support can further be simplified. As such, the combination of the values of the outer diameter of each photoelectric conversion element and the light gathering ratio x is important in decreasing the number of photoelectric conversion elements and reducing the amount of silicon used for generating an electric power of 1 W.
When the outer diameter of each photoelectric conversion element is smaller than 0.5 mm, the number of necessary photoelectric conversion elements becomes unduly large though the used amount of silicon decreases. When the outer diameter of each photoelectric conversion element is greater than 2 mm, the used amount of silicon becomes unduly large though the number of necessary photoelectric conversion elements decreases.
When the light-gathering ratio is smaller than 2, the used amount of silicon cannot be reduced sufficiently. When the light-gathering ratio is greater than 8, the light-gathering efficiency becomes smaller than 80%, for example, and the performance lowers accordingly. By setting the light-gathering ratio x in the proper range, the invention makes it possible to make the light-gathering efficiency greater than 80% or even 90%.
According to the invention, by setting the values of the outer diameter of each photoelectric conversion element and the light-gathering ratio x in the above-mentioned ranges, such a remarkable advantage is achieved that both of the number of necessary photoelectric conversion elements and the amount of silicon to be used for generating an electric power of 1 W can greatly be decreased to ⅕ to {fraction (1/10)} of those in the third prior art technique.
In a photovoltaic apparatus in which amorphous silicon photoelectric conversion elements are used and light is gathered at a light-gathering ratio in the above-mentioned range, the temperature of the photoelectric conversion elements can be increased to 40xc2x0 C. to 80xc2x0 C., which is higher than in the case of using an amorphous silicon, thin-plate photoelectric conversion element. This makes it possible to suppress deterioration of the amorphous silicon photoelectric conversion element and to thereby elongate the life of the photovoltaic apparatus.
In the invention it is preferable that the photoelectric conversion elements have a pn junction in such a manner that the second semiconductor layer of one conductivity type having a wider optical band gap than the first semiconductor layer having the other conductivity type does is formed outside the first semiconductor layer (see FIG. 14).
In the invention it is preferable that the photoelectric conversion elements have a pin junction in such a manner that the first semiconductor layer having one conductivity type, an amorphous intrinsic semiconductor layer, and an amorphous second semiconductor layer of the other conductivity type having a wider optical band gap than the first semiconductor layer does are arranged outward in this order (see FIGS. 15 and 16).
In the invention it is preferable that the first semiconductor layer and the second semiconductor layer are made of n-type silicon and p-type amorphous SiC, respectively.
In the invention it is preferable that the n-type silicon of which the first semiconductor layer is made is n-type single-crystal silicon or n-type microcrystalline (xcexcc) silicon.
According to the invention, a pn or pin heterojunction window structure is formed by different kinds of amorphous semiconductors. The optical band gap of the second semiconductor layer located on the light incidence side and made of a window material is set wider than that of the inside first semiconductor layer. With this measure, the optical absorption coefficient of the second semiconductor layer is made small, that is, light is not much absorbed by the second semiconductor layer, whereby the electron-hole recombination rate of the surface layer is decreased and the optical absorption loss is reduced. Further, the sensitivity on the shorter-wavelength side is increased to attain wide gap window action. The energy conversion efficiency can be increased as a result of those effects.
In particular, the pin junction structure can introduce much optical energy to the intrinsic semiconductor layer (i layer) as a photoelectromotive force generating layer and increase the sensitivity in the shorter-wavelength side to attain wide gap window action. The invention makes it possible to perform a far better energy converting operation than the particle according to the third prior art technique does in which the n-type silicon skin is formed outside the p-type silicon sphere.
The i layer of a photoelectric conversion element having a pin junction plays roles of forming electron-hole pairs through light absorption there, generating a photocurrent, and transporting it. The p layer and the n layer play a role of collecting photocarriers by fixing the Fermi level at a position close to the valence band or the conduction band and generating an internal electric field for carrying electrons and holes generated in the i layer to the two electrodes.
A fourth aspect of the invention provides a spherical semiconductor particles mass-producing apparatus comprising a crucible for storing a semiconductor; heating means for heating and melting the semiconductor in the crucible; a nozzle for dropping a molten semiconductor coming from the crucible; and vibrating means for vibrating the molten semiconductor and thereby converting, in a vapor phase, the dropping molten semiconductor into spherical particles having uniform particle diameters.
According to the invention, a semiconductor is stored in the crucible and melted by the heating means. A resulting molten semiconductor is dropped from the nozzle and vibrated by the vibrating means. As a result, the molten semiconductor dropping from the nozzle is converted into spherical particles in a vapor phase. The spherical particles have approximately a constant diameter. In this manner, spherical semiconductor particles can easily be mass-produced by a simple operation. The term xe2x80x9cvapor phasexe2x80x9d may be air or an inert gas of Ar, N2, or the like, and even encompasses the vacuum.
A molten semiconductor dropping from the nozzle is in liquid form rather than solid line form. Therefore, a large number of spherical semiconductor particles can easily be manufactured at high speed and hence in a short time. For example, the invention makes it possible to manufacture spherical semiconductor particles by dropping a molten semiconductor from the nozzle at a rate of 1 cm/s to 1 m/s, which is much higher than in the above-described prior art technique.
In the invention it is preferable that the mass-producing apparatus further comprises means for pressuring the molten semiconductor in the crucible.
In the invention it is preferable that the pressurizing means is a gas source for supplying an inert gas having a pressure higher than atmospheric pressure to a space over the semiconductor in the crucible.
In the invention it is preferable that a pressure of a space with which an outlet of the nozzle communicates is selected to be lower than that of a space over the semiconductor in the crucible does.
In the invention it is preferable that a plurality of the nozzles are provided and each of the nozzles has an inner diameter of 1xc2x10.5 mm and a length of 1 mm to 100 mm.
In the invention it is preferable that each of the nozzles has a length of 5 mm to 10 mm.
According to the invention, a molten semiconductor in the crucible may be pressurized by means of a gas or a liquid or by using a piston or the like. The pressurized molten semiconductor drops from the crucible. For example, to pressurize a molten semiconductor by means of a gas, an inert gas of Ar, N2, or the like having a pressure higher than atmospheric pressure is supplied from a gas source to the space over the semiconductor in the crucible. Alternatively, the pressure of the space under the outlet of the nozzle may be set lower than the pressure of the space over the semiconductor in the crucible so that a molten semiconductor coming from the crucible drops from the nozzle. Setting the nozzle inner diameter and length at 1xc2x10.5 mm and 1 mm to 100 mm (preferably 5 mm to 10 mm), respectively, can cause a molten semiconductor to drop from the nozzle at a constant flow rate, for example, by means of the pressure of the pressuring means without dropping at an unduly high flow rate because of its own weight. This makes it possible to accurately produce spherical particles having uniform particle diameters.
In the invention it is preferable that the heating means comprises an induction heating coil provided in the vicinity of the crucible and a high-frequency power source for energizing the induction heating coil.
In the invention it is preferable that the heating means is resistive heating means for heating the crucible.
That is, the heating means for heating a semiconductor in the crucible may have a configuration for induction heating including an induction heating coil and a high-frequency power source, or may be resistive heating means such as an electric heater that heats the crucible by Joule heat.
In the invention it is preferable that the vibrating means has a vibration frequency of 10 Hz to 1 kHz.
With this setting, a molten semiconductor is converted into spherical semiconductor particles having uniform particle diameters, which enables mass-production of spherical semiconductor particles.
In the invention it is preferable that the vibrating means applies sound waves or ultrasonic waves to the dropping molten semiconductor and thereby vibrate the dropping molten semiconductor.
According to the invention, applying sound waves or ultrasonic waves to a dropping molten semiconductor makes it possible to accurately convert the dropping molten semiconductor into spherical particles having uniform particle diameters.
In the invention it is preferable that the nozzle is vibratory, and the vibrating means vibrates the nozzle by reciprocating.
In the invention it is preferable that the vibrating means drives the nozzle so that an outlet of the nozzle vibrates in a direction perpendicular to the axial line of the nozzle at an amplitude A that is smaller than xc2xd of an outer diameter D1 of particles to be formed.
In the invention it is preferable that the vibrating means vibrates the nozzle along the axial line of the nozzle.
According to the invention, the nozzle is vibrated so as to reciprocate at least in the vicinity of its outlet, which makes it possible to accurately produce spherical semiconductor particles having uniform particle diameters. Vibrating the outlet of the nozzle in the direction perpendicular to its axial line at the amplitude A that is smaller than xc2xd of the outer diameter D1 of particles to be formed so as to have uniform particle diameters (i.e., A less than Dxc2xd) makes it possible to accurately produce spherical particles having the outer diameter D1. Alternatively, the nozzle may be vibrated along its axial line, that is, in the vertical direction. This also makes it possible to produce spherical particles having uniform diameters (D1).
In the invention it is preferable that the vibrating means is pressure varying means for varying a pressure of a space over the semiconductor in the crucible.
In the invention it is preferable that the vibrating means comprises a diaphragm provided so as to communicate with the space over the semiconductor in the crucible, and a driving source for reciprocating the diaphragm.
In the invention it is preferable that the vibrating means comprises a driving chamber that is connected to the space over the semiconductor in the crucible, and a driving source for oscillating a pressure inside the driving chamber.
According to the invention, the vibrating means for vibrating a molten semiconductor is pressure varying means for varying the pressure of the space over the molten semiconductor in the crucible by applying gas pressure to the space. The pressure varying means may be implemented by a diaphragm and a driving source (e.g., a motor and a crank) for reciprocating the diaphragm, or by a driving chamber and a driving source for varying the capacity of the driving chamber and thereby oscillating the pressure inside the driving chamber.
In the invention it is preferable that the vibrating means vibrates the crucible.
According to the invention, to vibrate a molten semiconductor, the crucible in which the molten semiconductor is stored temporarily may be vibrated by a driving source.
In the invention it is preferable that the mass-producing apparatus further comprises Lorentz force generating means for exerting Lorentz force on the molten semiconductor dropping from the nozzle and thereby forming particles through a pinch effect of decreasing a cross-section of the molten semiconductor.
According to the invention, a current is caused to flow through a conductive molten semiconductor dropping from the nozzle and an AC magnetic field is developed around the molten semiconductor, whereby Lorentz force is exerted on the liquid column molten semiconductor and a resulting pinch effect decreases its cross-section. This makes it possible to accurately convert a molten semiconductor dropping from the nozzle into spherical particles having uniform diameters.
A fifth aspect of the invention provides a spherical semiconductor particles mass-producing apparatus comprising a crucible for storing a semiconductor temporarily; heating means for heating and melting the semiconductor in the crucible; a nozzle for dropping a molten semiconductor coming from the crucible; vibrating means for vibrating the molten semiconductor and thereby converting, in a vapor phase, the dropping molten semiconductor into spherical particles having uniform particle diameters; and crystallizing means for heating liquid or solid particles dropping from the nozzle in the vapor phase to control a cooling rate thereof and thereby converting the particles into single-crystal or polycrystalline particles.
A sixth aspect of the invention provides a spherical semiconductor particles mass-producing apparatus comprising crystallizing means for heating liquid or solid particles existing in a vapor phase and thereby converting the particles into single-crystal or polycrystalline particles.
According to the invention, liquid or solid particles dropping from the nozzle is heated and re-melted by the crystallizing means, whereby the particles are converted into single-crystal or polycrystalline particles in the vapor phase.
Particles to be heated may be molten (i.e., liquid) semiconductor particles or solid particles formed by cooling of molten semiconductor particles dropping from the nozzle, or even particles obtained by pulverizing or crushing a bulk semiconductor.
In the invention it is preferable that the crystallizing means is a laser source for applying laser light to the particles.
In the invention it is preferable that the crystallizing means is a radiation heat source provided adjacent to a passage of the particles, for heating the particles by radiation heat.
In the invention it is preferable that the crystallizing means heats the particles so that the cooling rate of the particles has a gentle profile, to thereby prevent development of cracks in the particles and prevent the particles from becoming amorphous.
According to the invention, the crystallizing means may be a laser source or a radiation heat source for generating radiation heat. The temperature decrease rate of particles when they are cooled is lowered by the crystallizing means, whereby development of cracks in the particles is prevented and the particles are prevented from becoming amorphous. As a result, single-crystal or polycrystalline spherical particles are formed reliably.
A seventh aspect of the invention provides a spherical semiconductor particles mass-producing apparatus comprising a crucible for storing a semiconductor temporarily; heating means for heating and melting the semiconductor in the crucible; a nozzle for dropping a molten semiconductor coming from the crucible; vibrating means for vibrating the molten semiconductor and thereby converting, in a vapor phase, the dropping molten semiconductor into spherical particles having uniform particle diameters; crystallizing means for heating liquid or solid particles dropping from the nozzle in the vapor phase to control a cooling rate thereof and thereby converting the particles into single-crystal or polycrystalline particles; and diffusing means for causing crystalline semiconductor particles of one conductivity type to pass through a passage in a material gas containing atoms or molecules with which the crystalline semiconductor particles are to be doped, and thereby forming a surface layer of the other conductivity type on each of the crystalline semiconductor particles.
The invention provides a spherical semiconductor particles mass-producing apparatus in which crystalline semiconductor particles of one conductivity type are passed through a passage in a material gas containing atoms or molecules with which the crystalline semiconductor particles are to be doped, to form a surface layer of the other conductivity type on each of the crystalline semiconductor particles.
According to the invention, each of crystalline semiconductor particles of one conductivity type (e.g., p-type) can be formed with a surface layer of the other conductivity type (e.g., n-type) by a simple operation according to a gas diffusion method or a solid-state diffusion method. The gas diffusion method is a technique that a doping impurity is sent, in gas form, to a high-temperature silicon surface. The solid-state diffusion method is a technique that a diffusion agent containing an impurity is deposited on a silicon surface and then the silicon is heat-treated at a high temperature.
In the invention it is preferable that the passage extends in a vertical direction and surface layer diffusion is performed as the crystalline semiconductor particles drop through the passage.
An exemplary case of forming a diffusion layer on a surface of each silicon sphere by the diffusion method is to forma shallow n-type diffusion layer in each of p-type silicon spheres by the gas diffusion method will be described below. A gas of P2O5, POCl3, PH3, or the like is used as a diffusion source. First, an inert gas containing the diffusion source and a slight amount of hydrogen is introduced to a diffusion layer forming space that is adjacent to from the laser light application space and is separated from the latter in terms of atmosphere, and the diffusion layer forming space is filled with the inert gas. After being re-crystallized with high quality by illumination with laser light, p-type silicon spheres pass through the diffusion layer forming space from its top portion to its bottom portion while being kept at a high temperature. As the p-type silicon spheres pass through the diffusion layer forming space, each of them is formed, over the entire surface, with an n-type diffusion layer at a depth that is necessary for each silicon sphere to function as a solar cell. This step can be executed continuously so as to produce a large number of silicon spheres each formed with a surface layer by introducing the inert gas continuously and properly controlling the gas atmosphere of the diffusion layer forming space.
In the invention it is preferable that the crystalline semiconductor particles on which a diffusion agent is deposited bypassing through the passage are heated to form thereon a surface layer having a desired thickness.
According to the invention, p-type silicon spheres, for example, are caused pass through the diffusion layer forming space from its top portion to its bottom portion and, during that course, each of them is formed, over the entire surface, with a shallow n-type diffusion layer. Then, a number of resulting silicon spheres are put on a container made of quartz or the like and again subjected to a heat treatment. As a result, n-type diffusion layers having a desired thickness are formed.
In the invention it is preferable that the semiconductor is silicon.
According to the invention, the invention may be practiced for another semiconductor.
An eighth aspect of the invention provides a photoelectric conversion element comprising a plurality of semiconductor layers formed by the above-described mass-producing apparatus.
A ninth aspect of the invention provides a photovoltaic apparatus comprising a plurality of photoelectric conversion elements described above.
A tenth aspect of the invention provides a spherical semiconductor particles mass-producing method comprising the steps of heating and melting a semiconductor; dropping a molten semiconductor in a vapor phase; and vibrating the molten semiconductor.
An eleventh aspect of the invention provides a spherical semiconductor particles mass-producing method comprising the step of heating and re-melting, in a vapor phase, dropping semiconductor particles and thereby converting the semiconductor particles into single-crystal or polycrystalline semiconductor particles.
In the invention it is preferable that the mass-producing method further comprises the step of performing diffusion in a gas containing a composition with which the single-crystal or polycrystalline semiconductor particles are to be doped.
According to the invention, it is made possible to easily manufacture a photovoltaic apparatus using those photoelectric conversion elements composed of spherical semiconductor particles. The photovoltaic apparatus using such spherical photoelectric conversion elements is the first to generate a high electric power per unit area opposed to a light source using as small an amount of single-crystal or polycrystalline semiconductor material as possible. The photoelectric conversion elements may be made of not only a single-crystal or polycrystalline material but also an amorphous material.
According to the invention, the photoelectric conversion efficiency can be increased by introducing a microcrystalline (xcexcc) semiconductor layer, which has high conductivity, between the first semiconductor layer and a pin junction layer. An amorphous pin junction layer or a heterojunction of an amorphous pin junction layer and the second semiconductor layer can enable efficient collection of photocarriers and reduce the recombination loss of photocarriers.
The temperature of the amorphous semiconductor is increased to 40xc2x0 C. to 80xc2x0 C. when receiving light that is reflected by the inside surface of the recess of the support. This suppresses deterioration of the photoelectric conversion characteristic and hence is advantageous. Since each photoelectric conversion element has an approximately spherical shape, increase of the incident optical energy per unit area for receiving direct light or reflection light is suppressed, which also suppresses deterioration of the photoelectric conversion characteristic.
In the invention it is preferable that the first semiconductor layer is a direct gap semiconductor layer.
In the invention it is preferable that the direct gap semiconductor layer is made of a semiconductor selected from the group consisting of InAs, GaSb, CuInSe2, Cu(InGa)Se2, CuInS, GaAs, InGaP, and CdTe.
According to the invention, employing, as the inside first semiconductor layer, a direct gap semiconductor which easily absorbs light makes it possible to obtain sufficiently high electron and hole transition probabilities, which also contributes to increase of the photoelectric conversion efficiency.
In the invention it is preferable that a plurality of the supports each having peripheral portions extending outward are arranged adjacent to each other, and that part of the first conductor in the peripheral portion of one support of each pair of supports adjacent to each other and part of the second conductor in the peripheral portion of the other are laid one on another and electrically connected to each other.
In the invention it is preferable that the peripheral portion has upward projections or downward projections, and that the upward projection or downward projection of one support of each pair of supports adjacent to each other and the upward projection or downward projection of the other are brought into contact with and electrically connected to each other.
According to the invention, where part of the first conductor in a peripheral portion of one support of each pair of supports adjacent to each other among a plurality of supports mounted with photoelectric conversion elements and part of the second conductor in a peripheral portion of the other are laid one on another and electrically connected to each other, photoelectromotive forces, generated by the photoelectric conversion elements, of the respective supports are connected to each other in series. This makes it possible to output a desired high voltage.
According to the invention, where an upward projection and a downward projection, upward projections, or downward projections of peripheral portions of each pair of supports adjacent to each other are electrically connected to each other (see FIGS. 12 and 13), the recesses of each support can be made closer to each other and as many recesses and photoelectric conversion elements as possible can be arranged in a limited area.
The invention makes it possible to greatly reduce the used amount of photoelectric conversion element material (in particular, expensive silicon) and to simplify the step of connecting the photoelectric conversion elements to the support by decreasing the number of photoelectric conversion elements, to thereby increase the productivity and reduce the cost. In particular, the use of the photoelectric conversion elements according to the invention makes it possible to realize a manufacturing method capable of saving resources and energy. Sunlight or the like is reflected by the surface of the first conductor or a coating formed thereon that constitutes the inside surface of each recess of the support and resulting reflection light shines on the photoelectric conversion element. In this manner, incident light can be utilized effectively. The first conductor or a coating formed thereon serves to not only reflect incident light but also guide currents (the first conductor is connected to the second semiconductor layers of the respective photoelectric conversion elements). Having a simple structure, the support is superior in productivity.
In particular, by setting the outer diameter of each photoelectric conversion element at 0.5 mm to 2.0 mm (preferably 0.8 mm to 1.2 mm) and the light-gathering ratio x at 2 to 8 (preferably 4 to 6), the invention provides a remarkable advantage that both of the amount of silicon to be used for generating an electric power of 1 W and the number of necessary photoelectric conversion elements can greatly be decreased to ⅕ to {fraction (1/10)} of those in the third prior art technique. Reducing the used amount of silicon makes it possible to realize a photovoltaic apparatus at a low cost. Decreasing the number of photoelectric conversion elements and thereby simplifying the step of electrically connecting the photoelectric conversion elements to the support increase the productivity, which also contributes to realization of a low-cost photovoltaic apparatus.
Therefore, the invention makes it possible to provide a highly reliable, highly efficient photovoltaic apparatus.
A pn or pin junction is formed by setting the optical band gap of the outside amorphous second semiconductor layer wider than that of the center-side first semiconductor layer. With this measure, light is not much absorbed by the light-incidence-side second semiconductor layer made of a window material, whereby the recombination rate of the surface layer is decreased and wide gap window action is attained. The energy conversion efficiency can be increased as a result of those effects.
Inserting a microcrystalline (xcexcc) semiconductor layer having high conductivity between the center-side first semiconductor layer and the outside pin junction layer makes it possible to increase the energy conversion efficiency.
The invention also makes it possible to increase the energy conversion efficiency by using a direct gap first semiconductor layer.
Further, the invention facilitates manufacture of photoelectric conversion elements.
The invention makes it possible to mass-produce spherical semiconductor particles having uniform particle diameters by a simple operation by dropping, from the nozzle, a semiconductor that has been melted in the crucible and vibrating the molten semiconductor. Particles thus produced can easily be converted into single-crystal or polycrystalline particles in a vapor phase. And each crystalline semiconductor particle can easily be formed with a surface layer though impurity doping.